When the Mexican-American War concluded in 1848, those living in the Rio Grande Valley suddenly found themselves on the US side of the border. Most received US citizenship and decided to stay. By the late 1880s, more than 80% of the population of the Valley—which by then had become a four-county region in South Texas—still identified as “Mexicans” on the US census. They retained the small plots of land that they’d received through Spanish land grants and passed them on from...